it can be in a paste form as well, yes you apply, then solder, obviously it cleans the surface, what is the point of cleaning the surface after you have already (tried, and probably failed) soldered.@afiser, i have seen this video before, they are referring something liquid as flux.
But to my local store they are pointing me something hard and looks like crystal. what is that truly?
Also, should I apply flux first on the Motherboard with brush? then go for soldering?